A global navigation satellite system (GNSS) is a satellite-based navigation system used to determine a position (or a geographic location) of a receiver on Earth. The Global Positioning System (GPS) is an example of a GNSS. The GPS includes a constellation of 24 satellites (21 GPS satellites and three spare satellites) that orbit the Earth at 10,600 miles above the Earth. The satellites are spaced so that from any point on Earth, at least four satellites will be above the horizon. The satellites enable a receiver to determine a position (e.g., a geographic location) of the receiver. In commercial applications, the accuracy of a determined position can vary between 100 meters to 10 meters from an actual position of a receiver. In other applications requiring more precision—e.g., military applications, survey applications, or the like—the accuracy of a determined position can be within one meter from an actual position of a receiver.
Each satellite includes a computer, an atomic clock, and a radio. With knowledge of its own orbit and clock, each satellite periodically broadcasts its position and time. Each satellite transmits signals (referred to herein as “satellite signals”) at precise intervals. On the ground, a receiver includes a computer that converts information received in the satellite signals into data including position, velocity, and time estimates. Using the data received from a given satellite, a receiver can calculate a position of the given satellite, and the distance between given satellite and the receiver. Accordingly, the receiver can then use triangulation or other techniques to determine a position of the receiver by obtaining similar information from a number of other satellites.